A fuel cell is an electricity generator which converts chemical energy of a fuel into electric energy by electrochemical reaction in a stack, which can be used to supply power for industrial, domestic and vehicle applications, as well as for small electronic equipment such as portable devices. Recently, the application field of fuel cells as highly efficient clean energy sources gradually extends.
FIG. 1 shows a unit cell of a general fuel cell.
As can be seen from FIG. 1, the unit cell of the general fuel cell includes a membrane electrode assembly (MEA, 10) in the innermost part thereof, and the membrane electrode assembly 10 includes a polymer electrolyte membrane 11 to allow protons to flow, and catalyst layers, that is, an air electrode (cathode, 12) and a fuel electrode (anode, 13) which are coated on opposite surfaces of the polymer electrolyte membrane 11 to allow hydrogen and oxygen to react.
In addition, the gas diffusion layer (GDL, 20) is stacked on an outer part of the membrane electrode assembly 10, that is, an outer part of the membrane electrode assembly 10 where the air electrode 12 and the fuel electrode 13 are disposed, and a separator 30 provided with a flow field to supply a fuel and discharge water produced by reaction is disposed outside the gas diffusion layer 20.
In this case, the gas diffusion layer 20 is obtained by forming a fine porous layer 22 on one or two surfaces of a carbon fiber support 21 generally including a porous carbon paper.
In addition, the carbon fiber support 21 generally includes carbon fibers and a polytetrafluoroethylene-based hydrophobic substance. For example, the carbon fiber may take the form of carbon fiber cloth, carbon fiber felt or carbon fiber paper.
In addition, the fine porous layer 22 may be formed by preparing a mixture of a carbon powder such as acetylene black carbon or Black Pearls carbon and a hydrophobic agent such as polytetrafluoroethylene (PTFE) and applying the mixture to one or two surfaces of the carbon fiber support 21 depending on desired application.
Meanwhile, oxidization of hydrogen occurs at the fuel electrode 13 to produce a hydrogen ion (proton) and an electron, which move to the air electrode 12 via the electrolyte membrane 11 and a wire, respectively, while electrochemical reaction of the hydrogen ion and electron moved from the fuel electrode 13 with oxygen in the air occurs at the air electrode 12 to produce water and, at the same time, electric energy based on flow of the electron.
The gas-phase reactive gas supplied to the fuel cell and liquid-phase product water produced by chemical reaction are moved via the gas diffusion layer 20 from the membrane electrode assembly 10 and the separator 30.
In this case, the liquid-phase product water is moved through difference in capillary pressure in the gas diffusion layer 20 from the fine pore layer 22 to the carbon fiber support 21. Accordingly, the product water passes through the fine pore layer 22 and then moves in the carbon fiber support 21 via a selective channel having a greater difference in capillary pressure. That is, the product water tends to selectively move only through the channel facilitating movement, rather than through random channels in the gas diffusion layer.
As a result, the product water is concentrated on the selected channel, thus resulting in a problem in which the channel overflows with water and is thus clogged. The surrounding also overflows with water and the pores of the gas diffusion layer 20 are filled with water, disadvantageously blocking transfer of reactive gas and causing deterioration in cell performance (flooding).